Huge natural gas resources economically not viable for transportation to remote markets, diminishing limited oil resources, and increasing demand for clean fuels make the development of natural gas to liquid fuels conversion inevitable. Fischer-Tropsch synthesis (hereinafter: FT synthesis) is the most viable method for converting natural gas to liquid fuels. In this process, the natural gas is first converted to syngas by steam reforming and/or partial oxidation. Then the syngas is converted to long chain hydrocarbons, in the presence of cobalt-based or iron-based catalysts. The economy of the gas to liquid conversion process depends an the capital investment on the process, and more importantly on the average cost of the products. The production of syngas is the most expensive step in the conversion of natural gas to liquid fuels. Therefore, the Fischer-Tropsch process should be performed with the highest yield possible.
Depending an the operation conditions and composition of the catalysts used, the products range from C1 to C40 hydrocarbons. Methane and light gaseous hydrocarbons are undesirable products of FT synthesis and their recycle and conversion to syngas is costly. On the other hand, the heavy waxes also require hydro-cracking to middle distillates. Hydro-cracking results in more light gases. C5+ (weight fraction of pentane and heavier hydrocarbons) hydrocarbons, particularly middle distillates, need to be maximized. Many research efforts have focused an catalyst compositions, reactor systems, and operating conditions to improve the FT synthesis selectivity.
Production of significant amounts of methane, light gaseous hydrocarbons, and heavy waxes are of the major selectivity problem of the FT synthesis. By addition of proper promoters to the structure of the catalysts, researchers have tried to decrease methane selectivity, i.e., the yield of methane during FT-synthesis. Different transition metals, such as Ru, Re, Pt, Pd and Rh, alkaline metals, and rare earth oxides have been used to improve FT-synthesis activity and/or selectivity. Such promoters can decrease methane selectivity, increase the chain growth probability and enhance the yield of heavy waxes. Some other investigators have added α-olefins to FT synthesis reactor feed, to reduce the yield of methane and other light gaseous hydrocarbons. The major drawback of this method is separation of the α-olefins from the products to be recycled to the feed. Furthermore, one of the major problems of FT synthesis at high CO conversions is deactivation of the catalysts by oxidation and strong metal-support interactions in the presence of high partial pressures of water and also catalysts coking that are not resolved yet.
A two-stage apparatus for FT-synthesis has been proposed recently in U.S. Pat. No. 6,331,573 B1 and US 2002/0151605 A1. The first stage of FT-synthesis is performed using conditions in which chain growth probabilities are relatively low to moderate and the product of the reaction includes a relatively high proportion of low molecular weight olefins (C2–C8 olefins) and a relatively low proportion of high molecular weight waxes (C30+). The product from the first stage is fed into a second stage where the chain growth probabilities are relatively high. The temperatures of the first stage are higher than that of the second stage. Under these conditions wax and other paraffins produced in the first stage are relatively inert. Light olefins compete with heavier olefins for chain initiation to initiate fewer chains at C20+ so that a relatively large fraction in the C5-12 range is produced. In the first stage an iron-containing catalyst is used whereas in the second stage a cobalt-containing catalyst is used.
A high CO conversion ratio in the first stage may cause problems with deactivation of the catalysts in the first stage by oxidation and strong metal-support interactions and with high partial pressures of water. Furthermore, coking of the catalysts may cause serious problems.
GB 631 682 A discloses a two-stage process for synthesizing liquid fuels, wherein a mixture containing carbon monoxide and hydrogen is fed to a first reaction zone and wherein a feed mixture comprising the gaseous fraction from the first reaction zone and containing a ratio of hydrogen to carbon monoxide higher than that in the mixture fed to the first reaction zone is charged to a subsequent second reaction zone and wherein liquid products formed in the second reaction zone are recovered. In both stages water is intentionally added to promote the water gas shift reaction. The gaseous products of both stages are mixed and returned back to the first stage. The gases react in the two reaction zones at elevated temperatures of above 310° C.
U.S. Pat. No. 2,149,515 disclosed a multi-stage process for synthesizing liquid fuels, wherein one type of catalyst is used in all stages. This makes minimizing the yield of heavy waxed difficult. Use of three stages is not disclosed.
GB 515 037 A discloses a two-stage process for synthesizing liquid fuels with a focus on the production of mostly olefins. In this process the gases react substantially at atmospheric pressures, namely in the range between 0 and 10 atm. Products from the first stage are not separated before being charged to the second stage. An iron-based catalyst is used in the second stage.